Nailing the truth about the elusive convict fish proves tougher than expected.

Why would an 83-year-old marine biologist famous for up-close work with sharks turn her attention to a fish most people have never seen or even heard of? Because it behaves like no other sea creature Eugenie Clark has encountered in more than six decades of research. So secretive is the so-called convict fish (named for its stripes) that after seven years of study Clark still doesn't know how adults bear their young, or even what they eat. And those are just some of the mysteries.

Clark's latest passion was ignited in 1998 off the coast of Papua New Guinea, where she and a team of research divers first encountered convict fish. The divers saw clouds of tiny young fish vanish into a hole in the coral reef. They also noticed a serpent-like head (later discovered to be an adult) poking through the opening. Clark was hooked. "What were these fish, we wondered. I couldn't identify them."

After carefully studying a handful of specimens in her lab, Clark identified the fish as Pholidichthys leucotaenia. More often seen in aquariums than in the wild, they are popularly called convict blennies or engineer gobies. But DNA analyses strongly indicate that this fish is not related to the blenny or goby clans—nor to any other known fish family.

Full of questions, Clark's team headed to the Solomon Islands, where it videotaped the early morning spectacle of juveniles exploding like slow-motion fireworks from openings in the seafloor and nearby coral reefs. By day, quarter-inch to four-inch-long juveniles swam as far as 165 feet (50 meters) from their burrows to gulp down plankton. Remarkably, at day's end the swarms returned to their holes —a unique behavior for larval fish, which generally float among plankton and drift for miles. "No other coral reef fish have babies that come home to their parents," says Clark.

While the young feed, adults clean house, sticking their heads out just enough to spew mouthfuls of sand or gravel that currents have swept into their tunnels. By nightfall, heaps of debris fan out around the openings. These "sand aprons" may serve as welcome mats that help guide the young home.

To peer inside the private lives of convicts, biologist and photographer Stephen Kogge rigged an endoscope with a tiny video camera and a long cable connecting to a monitor on board the team's research vessel. Though he compares manipulating the device to "operating a wet noodle," Kogge managed to steer his "burrow cam" inside a tunnel.

A labyrinth of chambers Clark estimates to be as long as 20 feet (six meters) opened off a twisting passage. Clusters of young fish literally hung in the maze, their heads attached to the roof by mucous threads so thin they're almost invisible in the water. Juveniles dangled all night. "It's the weirdest looking sight," says Clark. "We don't know why they attach themselves. Perhaps by staying still they conserve oxygen and energy."

Equally odd, adults can grow to nearly two feet, but never seem to leave the tunnel to eat. Yet they often took in mouthfuls of juveniles then spat them back out. "First I thought they were cannibals," says Clark. "Some damselfish eat dead or dying embryos to weed out the weaker links." But when she examined the adults' stomachs, she found only an oozy green slime. Were the adults eating the juveniles' wastes? Did the parents take offspring into their mouths to protect them from danger? Maybe the young fish regurgitate digested plankton into their parents. If so, Clark says, this would be the first example of fish offspring feeding adults.

Despite a battle with cancer, Clark remains immersed in her convict research. At the Mote Marine Laboratory in Sarasota, Florida, Clark is observing specimens in tanks equipped with mirrors and see-through burrows (made from wine bottles) to learn how adults get enough nutrients to survive and whether they lay eggs or give birth to live young. She hopes to strap on her scuba gear again, as she has for more than half a century, and take to the seas, looking for answers.

Eugenie Clarkwww.sharklady.comFor more than six decades Eugenie Clark has studied the wildlife of the world's oceans. Learn more about her long and illustrious career at this site, which features her résumé, a list of her publications, and some frequently asked questions about her life and work.

Mote Marine Laboratorywww.mote.orgLocated in Sarasota, Florida, this lab was founded by Eugenie Clark half a century ago. Today its research scientists study many aspects of marine biology, from aquatic toxicology to coral reef biology to nautical archaeology. The lab's website offers a virtual tour of an aquarium that is open to the public, as well as profiles of Mote's Center for Shark Research, Center for Coastal Ecology, and other programs.